1. Field of the Invention
This invention relates to dewatering sludges, particularly sludged derived from food processing. It also relates to separating fats and oils from proteinaceous materials. It further relates to preparing animal foods by separating unwanted components from aqueous sludges.
2. Review of the Prior Art
Biological sludges are herein defined as those sludges produced by conversion of carbonaceous, proteinaceous, and fatty matters and/or sugars and carbohydrates to biological solids (microorganisms including bacteria, fungi, protozoa, rotifers, and the like) by biological oxidation under facultative or aerobic conditions to create activated sludge or by biological reduction or fermentation under anaerobic conditions to create anaerobic sludge. Biological sludges include municipal or sewage sludge which is relatively easy to dispose of by land application or by drying and landfilling.
In fish canneries, slaughter houses, and poultry slaughter and processing plants, large amounts of food wastewaters, having high protein and fat contents, are produced. These wastewaters are commonly processed by: (a) pretreatment of the wastewaters in a dissolved air flotation process which is operated with or without chemical coagulants and flocculants and which produces a primary float sludge or skimmings and/or (b) biological oxidation under facultative or aerobic conditions or biological reduction or fermentation under anaerobic conditions to produce biological sludges. Addition of chemical coagulants and of flocculants in the dissolved air flotation process is preferred because they float additional proteins from the wastewater.
The float sludge produced by dissolved air flotation from poultry slaughtering operations, for example, is a complex substance which contains approximately 35-40% protein, 10% ash and fiber, and 40% fat on a dry basis and has a solids content ranging from 5% to 30% but normally averaging about 10-15%. It is similar to municipal or sewage sludge in many respects, such as having a low solids content of 2% to 8% by weight. The high fat content complicates the disposal process and increases its cost. If this float sludge is hauled in a tank truck to a land disposal site, spread on the land, and plowed under, the disposal cost is $30 to $50 or more for each 2,000 gallons of activated sludge. A poultry slaughtering plant handling 70,000 birds per day (17,500,000 birds per year) produces 2,200-2,500 gallons per day (500,000 gallons per year) of this float sludge which has a 12-15% solids content and costs 2-5.cent. per gallon for disposal. This yearly disposal cost for float sludge is therefore $11,000 to $12,500 per year.
A poultry processing plant, for example, handling 70,000 birds (averaging about four pounds per bird) per day produces activated biological sludge from aerobic processing of its partially purified wastewater, after the float sludge has been removed therefrom, in the amount of approximately 4,000 gallons per day at 1% solids and 800 gallons per day of thickened sludge at 3-5% solids after passage through a flotation thickener. The annual costs of disposing of this activated sludge is $4,000. Thus the combined costs of disposing of both float and activated sludges is $15,000 to $16,500 per year.
Many attempts have been made to develop improved processes for disposing of both float and biological sludges. A large proportion of these attempts have involved the production of either an animal food and/or a cheaper or better fertilizer. None of these prior art processes appears to be suitable for converting float sludge into an animal feed supplement because each lacks a means and a step for removing excess fat from the protein and other desirable components. Cattle can digest feeds containing considerable proportions of cellulose, but they can utilize feeds containing no more than about 4% fat. Swine can utilize feeds containing no more than about 6-8% fat, and poultry can digest feeds containing up to about 14% fat.
The excessive fat content of such waste organic materials as float sludge is a problem in several industries, in addition to poultry slaughtering. Rendering plants, for example, discharge wastewaters averaging 1,660 mg/l of oil and grease, according to "Development Document for Effluent Limitations Guidelines and New Source Performance Standards for the RENDERER Segment of the MEAT PRODUCTS AND RENDERING PROCESSING Point Source Category", EPA-440/1-74/031-d, Group I, Phase II, 1975. Dissolved air flotation was reported therein as being the single most effective means for grease removal.
It is known, however, to thicken float sludge with sawdust as an aid in cooking it in a rotating dryer. In addition, wood chips have been added for the same purpose and to scour the dryer, commonly termed the cooker. Admixing a decomposable bulking agent, such as wood chips, bagasse, etc., with activated sludge for composting is also now practiced on a large scale.
U.S. Pat. No. 2,739,064 describes the use of dried activated sludge, when finely ground, as a component of poultry feeds for supplying the animal proteins that are usually available from fish solids.
U.S. Pat. No. 2,861,877 teaches the filtering of sewage sludge into a layer of loose, porous cellulosic material (preferably peat moss) that partially de-waters the sludge. The resulting mass of sludge and moss is tilled when its moisture content is 50-70%. The operation is repeated five times so that, after further drying to a moisture content of about 35%, the product has composted during the 20-day operation and contains 3 to 5 parts by weight of sewage sludge for each part of peat moss.
U.S. Pat. No. 2,905,557 deals with disposal of garbage before the large-scale usage of garbage disposers and paperboard containers. It proposes the admixing of an edible absorbent material, such as ground corn or wheat, corn cobs, corn stalks, hay, straw, or news print, with the garbage for absorbing the fats and oils in the garbage, for lubricating the surfaces of a heated drying griddle, and for conserving the food value of the fat when the dried mixture is utilized as animal feed.
U.S. Pat. No. 3,549,010 relates to treatment of municipal sludges, particularly primary sedimentation sludge, by admixing the slimy, gelatinous material with a pumpable slurry formed from municipal garbage and trash to form a free draining mixture containing more than 50 percent of fibrous constituents and 5 percent of sludge. This mixture is readily incinerated.
U.S. Pat. No. 3,909,410 describes a method for treating sewage sludge by distributing it onto preheated aggregate pieces to form a coating thereon. The coated aggregate is continuously fed into a rotating contactor drum until the coating becomes a hard encrustation which is pulverized, dislodged, and collected as dust. The decoated aggregate is recycled.
U.S. Pat. No. 4,282,256 discloses a method for preparation of an animal feed supplement in particulate form by recovering animal feed values from fish cannery wastewater. The concentrated sludge, separated from the water in a flotation cell maintained under aerobic conditions, is dewatered, blended with an animal-edible carrier, and dried under vacuum. Suitable carriers include brewer's yeast residue, spent brewer's grain, nuts and oilseed press cake meals, cottonseed meal, chopped hays, alfalfa, coarse ground corn cobs, rice hulls, beet pulp, bagasse, and the like.
U.S. Pat. No. 4,406,795 relates to a process for treating the wastewaters of high protein and fat content (i.e., in slurry form) from slaughtering houses, canning industries, organotherapeutic drug production, and the like. The slurry is ground very fine, is heated to a temperature of 50.degree.-125.degree. C. during a period of up to 2 minutes, is maintained at this temperature for at least 2 minutes and preferably for 5-13 minutes, and is filtered at this temperature, during at least 4 minutes and preferably 8-15 minutes, to separate aggregated and dewatered grains from water and melted fat. The filtered grains are then dried.
None of these prior art processes, however, has successfully solved the problem of removing excess fat from a sludge produced from the wastewater of a food preparation plant and then utilizing the defatted sludge as a proteinaceous animal food supplement. The cost of disposing of such sludges and the value of such food supplements indicates that there is a nationwide and longstanding need for a suitable defatting and protein enriching process that can thereby produce a nutritional animal food supplement.